As well known, in paper converting industry a variety of types of machines and of processes exists for making paper tissues, paper towels and the like by producing stacks of interfolded sheets of a certain height.
They are obtained folding the sheets in an “interfolded” way, namely are folded into panels by overlapping at the same time a panel of a previous sheet with a panel of a following sheet. In this way, when pulling a sheet from the stack, at the moment of the use also a panel of the following sheet is pulled out, with consequent advantages for certain types of uses. Among the possible interfolding ways, stacks of Z interfolded sheets are known.
Machines are known that use one or more webs of paper, coming from one or more reels, that are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, the webs are cut into sheets by means of cutting rollers that interact alternatively, with relative counter-support rollers.
The overlap between two consecutive sheets occurs just after the cutting step and a sequence of overlapping and offset sheets come to the folding rollers from only one direction.
The interfolded stacking step is accomplished with the aid of folding elements, which in case of rollers with mechanical folding means, consist of mechanical clamps incorporated in the roller.
More precisely, the cut sheets proceed on a first roller that rotates at a first speed. Then, the pass on a second roller, called “overlap” roller, which rotates at a second speed less than the first. This way, owing to this speed difference a sheet travelling upstream is quicker and partially overlaps a sheet travelling downstream, slower, which is raised by means of air blows. The overlapping is carried out for about a third of the sheets sheet length, in order to allow the Z interfolding arrangement, after the passage through the interfolding rollers.
Between the overlapping roller and the interfolding rollers a conveying system is normally provided having opposite belts, adapted to convey the partially overlapped sheets. The two belts are counter-rotating and travel at the same speed, keeping the sheets pressed on one another.
The belts are opposite and consist of a plurality of parallel strings, which extend for all the sheet length of the rollers. To avoid that they loose the desired overlapped condition of a third of their sheet length, the belts must partially to penetrate each other, causing the sheets to deform.
Furthermore, the presence of opposite belts is a structural complication for the machine, thus affecting the encumbrance and the costs.